Neptune’s global temperature is doing something unexpected. Over the last 17 years, Neptune has experienced quite a dramatic climate change. And for once, humanity is not responsible, although we are not certain what is behind it.
As reported in The Planetary Science Journal, Neptune’s global temperature dropped 8 °C (14.4 °F) between 2003 and 2018, as the southern hemisphere of the planet slowly entered its summer period (which lasts about 40 years, like every season on Neptune). But then by 2020, the south pole alone became warmer – 11 °C (19.8 °F) warmer than it was in 2018.
“This change was unexpected,” lead author Dr Michael Roman, a postdoctoral research associate at the University of Leicester, said in a statement. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”
Neptune's average temperature is about –220 °C (–364 °F) and the little heat it has comes from internal processes. Neptune radiates roughly 2.61 times as much energy as it receives from the Sun. So following the changes of the season provides insights into the evolution of the planet’s atmosphere. These new findings upend some expectations.
For example, researchers have known about the fact that Neptune has a warm polar vortex (or at least warm for Neptune) compared to the rest of the planet, but not that it would experience these wild changes in temperature. It is unclear what the cause behind it is. Variation in chemical composition in the planet’s stratosphere, solar activity, or even weather patterns could all be among the reasons. We know Neptune’s storms have been pretty weird too lately.
“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” said co-author Glenn Orton, senior research scientist at Caltech’s Jet Propulsion Laboratory in the US.
Humanity has not visited Neptune up close since the Voyager 2 in 1989. All the observations of the planet’s temperature come from (or nearby) Earth. The VLT Imager and Spectrometer for mid-InfraRed (VISIR) on the Very Large Telescope in Chile is particularly suited for this task, and the team combined this data with observations from NASA's Spitzer in Space as well other telescopes in Chile and in Hawai’i.
“This type of study is only possible with sensitive infrared images from large telescopes like the VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so,” explained co-author Leigh Fletcher, a professor at the University of Leicester.
NASA’s JWST and other observatories coming online in the coming years will provide an even more detailed look at Neptune, meaning the mystery of its climate might soon be revealed.